The immune response to polysaccharide (PS) antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage, fine specificity, and avidity. Simple PS not conjugated to protein (such as Neisseria meningitidis group C (MCPS)) elicit a thymus-independent (TI) response. PS conjugated to proteins (such as MCPS coupled to tetanus toxoid, (MCPS-TT)), on the other hand, elicit a different type of response, termed thymus-dependent (TD). Previous analyses of anti-MCPS mAb reveal that VH gene family usage is dominated by VHJ558. Sequence comparisons support the conclusion that several germline VHJ558 genes are used in response to MCPS. The sequence of 1705.18, 1846.13 and 1863.5 are encoded by the same germline. The sequence for 1863.5, an IgM mAb, has more mutations in its VH region than 1705.18 and 1846.13, both IgG3 antibodies likely generated from the same clone as they came from the same fusion. Genes used by 3006.18 and 177.16 resemble those anti-Dextran of group 45.21.1. The 2055.5 mAb utilizes yet another VHJ558 germline gene. Sequence analysis of VL genes from the anti-MCPS mAb revealed that the VkOX1 light chain of the Vk4/5 family was the VL most commonly paired with the VHJ558 genes. The sequences of1705.18 and 1863.5 are similar to 26.4.1 an anti-dextran mAb and 2055.5 is identical to 26.4.1 but uses Jk4 instead of Jk2. Three mAb utilize two different germline genes belonging to the VHQ52 family. 1702.10 and 3079.6 utilize the VHOx1 heavy chain denoted VH101. The 1922.2 mAb utilizes the VHOx2 germline. The fine specificities of 1702.10 and 1922.2 mAb are the same although they utilize different VL genes.Other VH-VL pairs are seen in this response in a restricted fashion. The expression of VH3609-Vk23 correlates with reactivity to native MCPS. This fine specificity is only seen in anti-MCPS TI-2 mAb, not in the anti-C2 or anti-CP panel. The sequence of these mAbs are very similar to the 3609.3 germline sequence. The VL gene is similar to a Vk23 sequene specific for a spin-labeled dinitrophenyl hapten (DNP-SL). Sequence analysis in progress will determine if somatic mutation can account for the increased diversity and increased avidity . Previous mouse model studies in our laboratory indicated a developmental delay in the immune response to MCPS even when they were administered as a TD antigen; MCPS-TT. In order to understand whether this delay in the response to TD conjugates in neonatal mice is due to defective antigen presentation, TT specific T cell clones were generated from mice that were immunized with MCPS-TT. One of these T cell clones showed 2-3 fold higher proliferative response to MCPS-TT than TT in vitro, suggesting this T cell clone might be specific to a glycosylated moiety of TT. In future studies, these T cell clones will be used to identify the antigen presenting cells involved in the presentation of MCPS-TT conjugates to T cells.